1. Field of the Invention
The present invention relates to a SiGe heterojunction bipolar transistor (HBT) and, more particularly, to a SiGe HBT with a shallow out-diffused p+ emitter region.
2. Description of the Related Art
A bipolar transistor is a well-known structure that has an emitter, a base connected to the emitter, and a collector connected to the base. The emitter has a first conductivity type, the base has a second conductivity type, and the collector has the first conductivity type. For example, an npn bipolar transistor has an n-type emitter, a p-type base, and an n-type collector, while a pnp bipolar transistor has a p-type emitter, an n-type base, and a p-type collector.
When the emitter and base are formed from different semiconductor materials, such as silicon and germanium, respectively, the interface is known as a heterojunction. The heterojunction limits the number of holes that can be injected into the emitter from the base. Limiting the number of injected holes allows the dopant concentration of the base to be increased which, in turn, reduces the base resistance and increases the maximum frequency of the transistor.
FIG. 1 shows a cross-sectional view that illustrates an example of a prior-art SiGe heterojunction bipolar structure 100. As shown in FIG. 1, bipolar structure 100 includes a silicon-on-oxide (SOI) wafer 110, which has a silicon handle wafer 112, a buried insulation layer 114 that touches silicon handle wafer 112, and a single-crystal silicon substrate 116 that touches buried insulation layer 114. Silicon substrate 116, in turn, has a heavily-doped, p conductivity type (p+) buried region 120 and a heavily-doped, n conductivity type (n+) buried region 122.
As further shown in FIG. 1, bipolar structure 100 includes a single-crystal silicon epitaxial structure 130 that touches the top surface of silicon substrate 116. Epitaxial structure 130 has a very low dopant concentration, except for regions of out diffusion. For example, a number of p-type atoms out diffuse from p+ buried layer 120 into epitaxial structure 130, and a number of n-type atoms out diffuse from n+ buried layer 122 into epitaxial structure 130. In the present example, epitaxial structure 130 is a very lightly doped, n conductivity type (n−) region, excluding the regions of out diffusion.
Bipolar structure 100 also includes a number of shallow trench isolation structures 132 that touch epitaxial structure 130, and a deep trench isolation structure 134 that touches and extends through epitaxial structure 130 as well as silicon substrate 116 to touch buried insulation layer 114. Buried insulation layer 114 and deep trench isolation structure 132 form an electrically-isolated, single-crystal silicon region 136 and a laterally-adjacent, electrically-isolated, single-crystal silicon region 138.
In addition, bipolar structure 100 includes a lightly-doped, p conductivity type (p−) region 140 that extends from the top surface of silicon epitaxial structure 130 down through epitaxial structure 130 to touch p+ buried region 120, and a lightly-doped, n conductivity type (n−) region 142 that extends from the top surface of silicon epitaxial structure 130 down through epitaxial structure 130 to touch n+ buried region 122.
Bipolar structure 100 also includes a p conductivity type sinker region 144 that extends from the top surface of silicon epitaxial structure 130 down through epitaxial structure 130 to p+ buried region 120, and an n conductivity type sinker region 146 that extends from the top surface of silicon epitaxial structure 130 down through epitaxial structure 130 to n+ buried region 122.
Sinker region 144 includes a heavily-doped, p conductivity type (p+) surface region and a moderately-doped, p conductivity type (p) lower region, while sinker region 146 includes a heavily-doped, n conductivity type (n+) surface region and a moderately-doped, n conductivity type (n) lower region.
Further, bipolar structure 100 includes a SiGe epitaxial structure 150 that touches and lies over silicon epitaxial structure 130, a shallow trench isolation structure 132, and p− region 140. SiGe epitaxial structure 150 has a number of layers including a top layer 151 and a lower layer 152 that touches and lies below top layer 151.
Top layer 151 includes a single-crystal silicon region and a polycrystalline silicon region. Top layer 151 also has an out-diffused emitter region 153, and an outer region 154 that touches out-diffused emitter region 153. Out-diffused emitter region 153, which lies in the single-crystal silicon region, has a heavy dopant concentration and a p conductivity type (p+).
Outer region 154, which horizontally surrounds out-diffused emitter region 153, has a very low dopant concentration and, in the present example, an n conductivity type (n−). Lower layer 152, in turn, includes a single-crystal germanium region that touches the single-crystal silicon region of top layer 151, and a polycrystalline germanium region that touches the polycrystalline silicon region of top layer 151. Lower layer 152 also has a heavy dopant concentration and an n conductivity type (n+). Thus, the single-crystal germanium region has an n+ dopant concentration.
Bipolar structure additionally includes a SiGe epitaxial structure 155 that touches and lies over silicon epitaxial structure 130, a shallow trench isolation structure 132, and n− region 142. SiGe epitaxial structure 155 has a number of layers including a top layer 156 and a lower layer 157 that touches and lies below top layer 156.
Top layer 156 includes a single-crystal silicon region and a polycrystalline silicon region. Top layer 156 also has an out-diffused emitter region 158, and an outer region 159 that touches out-diffused emitter region 158. Out-diffused emitter region 158, which lies in the single-crystal silicon region of top layer 156, has a heavy dopant concentration and an n conductivity type (n+).
Outer region 159, which horizontally surrounds out-diffused emitter region 158, has a very low dopant concentration and, in the present example, an n conductivity type (n−). Lower layer 157, in turn, includes a single-crystal germanium region that touches the single-crystal silicon region of top layer 156, and a polycrystalline germanium region that touches the polycrystalline silicon region of top layer 156. Lower layer 157 also has a heavy dopant concentration and a p conductivity type (p+).
Bipolar structure 100 additionally includes an isolation structure 160 that touches SiGe epitaxial structure 150, and an isolation structure 162 that touches SiGe epitaxial structure 155. Isolation structures 160 and 162 are electrically non-conductive. Isolation structure 160 has an emitter opening 164 that exposes the single-crystal silicon region of top layer 151 of SiGe epitaxial structure 150, and a contact opening 166 that exposes the polycrystalline silicon region of top layer 151 of SiGe epitaxial structure 150. Similarly, isolation structure 162 has an emitter opening 170 that exposes the single-crystal silicon region of top layer 156 of SiGe epitaxial structure 155, and a contact opening 172 that exposes the polycrystalline silicon region of top layer 156 of SiGe epitaxial structure 155.
Bipolar structure 100 further includes a heavily-doped, p conductivity type (p+) polysilicon structure 180 that touches isolation structure 160 and extends through emitter opening 164 to touch the p+ out-diffused emitter region 153 of SiGe epitaxial structure 150. Bipolar structure 100 also includes a heavily-doped, n conductivity type (n+) polysilicon structure 182 that touches isolation structure 162 and extends through emitter opening 170 to touch the n+ out-diffused emitter region 158 of SiGe epitaxial structure 155.
P+ polysilicon structure 180 and p+ out-diffused emitter region 153 form the emitter, the remaining portion of SiGe epitaxial structure 150 forms the n-type base, and the combination of p+ buried region 120, p− region 140, and p-type sinker region 144 form the collector of a pnp SiGe heterojunction bipolar transistor (HBT) 190.
In addition, n+ polysilicon structure 182 and n+ out-diffused emitter region 158 form the emitter, the remaining p-type portion of SiGe epitaxial structure 155 forms the p-type base, and the combination of n+ buried region 122, n− region 142, and n-type sinker region 146 form the collector of an npn SiGe HBT 192.
During an anneal in the fabrication of HBT 190 and HBT 192, p-type atoms in p+ polysilicon structure 180 out diffuse into top layer 151 of SiGe epitaxial structure 150 to form p+ emitter region 153, and n-type atoms in n+ polysilicon structure 182 out diffuse into top layer 156 of SiGe epitaxial structure 155 to form n+ emitter region 158.
One of the drawbacks of HBT 190 and HBT 192 is that p+ out-diffused emitter region 153 is significantly larger and deeper than n+ out-diffused emitter region 158 due to the higher diffusion rate of p-type atoms, such as boron, when compared to the lower diffusion rate of n-type atoms, such as phosphorous.
In applications where the pnp and npn parameters are to be matched as closely as possible, the significantly deeper depth of p+ out-diffused emitter region 153 when compared to the depth of n+ out-diffused emitter region 158 poses a problem. One approach to reducing the variation in the depths is to form a thin oxide layer on the portion of the single-crystal silicon region of top layer 151 of SiGe epitaxial structure 150 that is exposed by emitter opening 164.
FIG. 2 shows a cross-sectional view that illustrates an example of a prior-art SiGe heterojunction bipolar structure 200. SiGe heterojunction bipolar structure 200 is similar to SiGe heterojunction bipolar structure 100 and, as a result, utilizes the same reference numerals to designate the elements that are common to both structures.
As shown in FIG. 2, SiGe heterojunction bipolar structure 200 differs from SiGe heterojunction bipolar structure 100 in that SiGe heterojunction bipolar structure 200 utilizes a p+ out-diffused emitter region 210 in lieu of p+ out-diffused emitter region 153. P+ out-diffused emitter region 210 is similar to p+ out-diffused emitter region 153, except that p+ out-diffused emitter region 210 is smaller and shallower than p+ out-diffused emitter region 153.
SiGe heterojunction bipolar structure 200 also differs from SiGe heterojunction bipolar structure 100 in that SiGe heterojunction bipolar structure 200 includes an oxide layer 212 that lies between and touches p+ out-diffused emitter region 210 of SiGe epitaxial structure 150 and p+ polysilicon structure 180.
P+ polysilicon structure 180 and p+ out-diffused emitter region 210 form the emitter, the remaining portion of SiGe epitaxial structure 150 forms the n-type base, and the combination of p+ buried region 120, p− region 140, and p-type sinker region 144 form the collector of a pnp SiGe heterojunction bipolar transistor (HBT) 214.
During the anneal that causes the atoms to out diffuse, oxide layer 212 is thin enough to allow p-type atoms to diffuse through from p+ polysilicon structure 180 into the top layer 151 of SiGe epitaxial structure 150 to form p+ emitter region 210, but thick enough to slow down the rate at which the atoms diffuse into the top layer 151 of SiGe epitaxial structure 150. As a result, the depth of p+ out-diffused emitter region 210 can be formed to be approximately the same as the depth of n+ out-diffused emitter region 158.
One of the drawbacks of SiGe heterojunction bipolar structure 200 is that SiGe heterojunction bipolar structure 200 has a significantly larger 1/f noise than SiGe heterojunction bipolar structure 100 due to the presence of oxide layer 212. In addition, next generation HBTs commonly use epitaxially-grown single-crystal silicon structures to form the emitters in lieu of polysilicon structures like polysilicon structure 180. However, an oxide layer like oxide layer 212 cannot be used with epitaxially-grown single-crystal silicon emitters to reduce the depth of the p+ out-diffused emitter region because single-crystal silicon cannot be epitaxially grown on oxide. Thus, there is a need for a SiGe HBT with a shallow p+ out-diffused emitter region which is approximately equal to the depth of the n+ out-diffused emitter region.